1. Field of the Invention
The present invention relates to a rake receiver with a flexible architecture suitable for demodulating direct sequence code division multiple access (CDMA) communications or other types of CDMA communications.
2. Description of the Related Art
Aspects of this disclosure relate to code division multiple access (CDMA) technology. CDMA technology is presently used in a number of wireless communication applications including cellular telephone networks. Other applications of CDMA technology include satellite communications, some global positioning communication systems and wireless LANs and WANs using CDMA modems. CDMA is a spread spectrum technique that spreads a signal over a frequency band to achieve more robust and efficient communication. Typically a signal transmitted using CDMA is spread in frequency with a pseudonoise (PN) code. A PN code generally is expressed as a characteristic set of binary valued chips defined over a range of frequencies spanning the bandwidth of the communication channel. Direct sequence spread spectrum (DSSS) modulation techniques multiply a signal to be transmitted with the PN code that identifies the user or communication link and transmits the resulting modulated signal. Using the spreading code to expand the spectrum of the signal is the code division part of CDMA communication. The multiple access aspect of CDMA reflects the fact that multiple users can use the same bandwidth at the same central frequency for communication.
Multiple users can communicate over the same frequency band of a CDMA link because each of the users is assigned a different PN code and that user's signals are modulated with their assigned PN code prior to transmitting the signals over the network. Receivers despread and detect a particular user's signal according to the assigned or expected PN code. PN codes are nearly orthogonal and so can be used to identify users present on a single channel using the same frequency band. Often the communication between a base station and a mobile handset is accomplished over two channels having similar bandwidth characteristics but separated in frequency so that full duplex communication can proceed. This might be true whether the network is used for cellular telephony or a wireless LAN or WAN application. Because these channels are separated in frequency, the same PN code can be used to spread the user's signals on the two non-overlapping bandwidths of the two channels.
In a typical cellular telephone network, both the base station and the handset transmit and receive signals. The transmitted signals may vary significantly in power and quality because of the physical characteristics and limitations of the base station and the handset. Received signals can be of varying amplitudes with varying levels of noise. In such a cellular network, the separation between the cellular base station and the handset can vary significantly, particularly since the handset may be in an automobile. The range of amplitude variations can be large. Variations in receiver and transmitter position exist in other applications but are pronounced in cellular telephone applications and mean that a receiver may need to recover signals whose magnitude varies considerably.
Another challenge of cellular telephony is that of multipath propagation. When the cellular telephone network is within an urban environment or other environments with significantly varying terrain, a signal travelling between a base station and a handset may reach the receiver directly or after having reflected off of a building or other objects. Different portions of the signal may propagate along significantly different paths and a number of these different paths may be received at a receiver at substantially the same time. Different multipaths experience different levels of attenuation, phase delay and can arrive at different angles. Wireless communication subject to multipath propagation exhibits significant variations in received signal characteristics for small changes in the relative position of the receiver. These rapid variations for small changes in position are known as Rayleigh fading due to the power distribution of the received signal under such conditions. Because it uses a spread spectrum, CDMA communication has natural advantages in reducing the problems associated with multipath propagation. Diversity reception within the CDMA system can be used to achieve still better performance.
Diversity reception refers to the process whereby multiple receive channels are used to recover different portions of a signal. In the case of multipath propagation, the receiver might include two independent receiving channels. The two independent receive channels might be assigned to the two strongest multipaths that arrive at the receiver. Processing is performed to align the two strongest multipaths and the two multipaths are added according to an appropriate weighting scheme. Overall signal strength and signal to noise properties are improved. The rake receiver architecture is especially well suited to performing this form of diversity reception.
For multipath discrimination to be possible in the CDMA system, there should be a minimum separation between the times when the multipath signals are received. The minimum separation should be at least one chip in the PN sequence. When there is at least one chip difference in the reception time between two multipath signals there will be at least one chip of offset between the embodied PN codes. In CDMA systems, the PN codes are selected so that two PN codes separated in time by at least one chip have very low cross correlation and so can be treated as independent signals and processed accordingly so that the signals can be combined. This minimum difference in reception time or chip duration of course varies for different systems but is such that in most urban settings appropriate delays will exist between multipaths to make diversity reception generally advantageous.
Mobile receivers such as handsets or terminals identify base stations according to pilot signals transmitted by base stations. Pilot signals have particular characteristics that make them particularly well suited to identify appropriate base stations for the mobile receiver's communication and to allow the mobile receiver to detect aspects of the base station's identification, system timing and control signals. Information from the pilot signal is used by the mobile receiver to synchronize to the timing of the base station. Pilot signals are used to distinguish between different base stations, thus enabling the mobile terminal to accurately identify a specific base station and establish communication. Receivers generally identify the strongest pilot signal as the appropriate one to acquire and initiate communications with the corresponding base station.
PN codes used in present implementations of CDMA communication are sufficiently close to orthogonal to identify a given signal traveling through different multipaths to a receiver. Because they are not fully orthogonal, PN codes are not generally adequate to separate different data streams for a single user. Orthogonal codes are used for that purpose. For example, the pilot signal is spread by a specific orthogonal code while the data and control streams for a user may be spread by other orthogonal codes. When the base station transmits user signals to a mobile receiver, the user signals are modulated depending on the specific communication scheme in use. The orthogonal coding used for different communication links is generally set by standard, vary among different applications, and are expected to continue to evolve over time.
Rake receivers are known in the art. Useful illustrations of conventional aspects of rake receivers are provided, for example, by U.S. Pat. No. 5,764,687 entitled “Mobile Demodulator Architecture for a Spread Spectrum Multiple Access Communication System,” which patent is hereby incorporated by reference in its entirety. Aspects of conventional rake receivers are discussed here with reference to FIG. 1, which illustrates aspects of U.S. Pat. No. 5,764,687. Further aspects of rake receivers are illustrated in U.S. Pat. No. 5,903,550 entitled “Method and System for Parallel Demodulation of Multiple Chips of a CDMA Signal.” This patent is similarly incorporated by reference in its entirety for its teachings regarding aspects of the implementation and operation of rake receivers.
The wireless channel changes dynamically over time. This change is slow enough to be considered constant over a sufficiently short duration of time conventionally known as the coherence time. This coherence time is a function of the channel parameters and the speed of the mobile terminal relative to the base station. Over a coherence time the receiver should be able to estimate the channel characteristics and coherently sum up all of the different multipath contributions from the rake fingers. Each rake finger should then be able to track the variations that occur over time for its assigned multipath component. For example, each rake finger usually includes the ability to track the timing of a received multipath signal and to adjust the rake finger's timing for data recovery as the delay of the assigned multipath varies. The rake finger may also track other characteristics of the multipath signal that vary with time.
FIG. 1 provides an overview of a conventional implementation of a rake receiver 10 for a CDMA communication system that includes three rake fingers 12, 14, 16 for demodulation and one rake finger used as a searcher 24. Each finger 12, 14, 16 consists of two despreaders, one for pilot processing and one for data demodulation. Once assigned to a multipath, the finger continues to receive and process the pilot signal as required. The rake receiver 10 coherently combines the three multipaths using estimates of the amplitude and phase of the three strongest paths. This is accomplished in the signal combiner 26 within the demodulator 22. Each multipath component of the signal is weighted, phase adjusted and delay-adjusted using channel estimates. The total signal is a coherent sum of the three strongest multipaths.
The FIG. 1 rake receiver includes an analog front end section 18 that receives RF signals from an antenna. The analog section 18 down-converts the received RF signal to IF (intermediate frequency) or baseband, where it is oversampled by the sampling clock 20. Usually this oversampling is relatively high, such as eight times the chip rate, to allow timing recovery techniques to synchronize the system. The digitized I (in phase) and Q (quadrature) signals are then presented to the demodulating block 22 where the searcher 24 starts to test different delays of the locally generated PN code (not shown) against the received pilot I and Q signals. The searcher 24 tests for energy at different delays for the target PN code. Once the searcher detects a sufficient level of energy for a given delay, the searcher declares a lock on the signal and establishes the delay for recovering that signal. The searcher 24 informs the controller 26 of the lock and that a candidate offset is available. The controller 26 then transfers the state information to one of the available fingers 12, 14, 16 to initiate a fine tracking algorithm and signal demodulation.
The analog front end 18 is operated under closed loop control. Gain control information is fed back from a gain control section 28 to provide automatic gain control feedback to the analog front end 18. Frequency control information is derived by a frequency control section 30 from the signals output by the rake fingers 12, 14, 16. Frequency control section 30 provides frequency information to the analog front end 18 as a feedback signal to maintain a coarse lock for frequency recovery.
Each rake finger 12, 14, 16, operating in conjunction with the analog front end, has the capability of tracking a specific assigned multipath and correcting for the effects of propagation, including phase rotation and frequency and timing drifts. Demodulation is accomplished using correlators within each of the fingers to provide a multipath component of the signal. After correction, the multipath components from the rake fingers are combined together to give a soft estimate of the received symbol. The signals and the soft estimate are passed on to a slicer or channel decoder that generates a final, hard symbol decision.
Fine tracking is performed within the fingers 12, 14, 16 and involves two processes. The first is timing tracking, where the timing tracking loop measures energy on two sides (early and late) of the sample that is assumed to be the correct sampling instant. The second process determines the difference in energy between these two side samples. The difference in energy is then filtered and processed to control a decimator that selects which samples are to be chosen from the incoming oversampled data stream. Thus the ideal sampling instant is fine-tuned in steps of chip period divided by the oversampling ratio. The decimated sample stream is then despread by the I and Q PN codes to generate the pilot samples (PI, PQ). Furthermore the orthogonal modulation is uncovered to generate the data samples (DI, DQ). As discussed above, I and Q are the inphase and quadrature components of the pilot and data sequences, respectively.
The pilot signal is used to estimate the rotation of the associated data vector caused by propagation and attenuation of the I and Q components of the signal. Prior to combining the outputs of the rake fingers, the outputs are de-rotated and weighted by their relative strength. Thus the combination performed by the section 26 is a coherent maximum ratio combining process. Finally, the symbols are presented to a de-interleaver/decoder circuit that deinterleaves the received data stream and further uses a decoder such as a Viterbi decoder to remove coding. The decoded data are then made available to higher layer communication protocols.